It is known to fabricate micro-hotplate structures on a silicon substrate. Such structures consist of a micro-heater embedded within a thin dielectric membrane, typically consisting of silicon dioxide and/or silicon nitride.
Such structures can be used for resistive gas sensors, by having electrodes on top, onto which a gas sensitive material is deposited. For example, U. Dibbern et. al, “A substrate for thin-film gas sensors in microelectronic technology,” Sensors and Actuators B, 1990 describes the design of a micro-hotplate using NiFe alloy as a heater material, in an oxy-nitride membrane. The device has electrodes on top and is used for gas sensing. Similarly M. Stankova et. al, “Detection of SO2 and H2S in CO2 stream by means of WO3-based micro-hotplate sensors” Sensors and Actuators B, 2004 describes micro-hotplates based on a polysilicon heater within an oxy-nitride membrane. M. Baroncini et. al., “Thermal characterization of a microheater for micromachined gas sensors,” describes a gas sensor with a micro-heater made from platinum.
Similarly, many such reports can be found in literature using micro-hotplate devices for gas sensors. Reference to some of these are given in I. Simon et. al, “Micromachined metal oxide gas sensors: opportunities to improve sensor performance,” Sensors and Actuators B (2001), and S. Z. Ali Et. Al, “Tungsten-Based SOI Microhotplates for Smart Gas Sensors” Journal of MEMS 2008.
Most of these reported devices are not fabricated in a standard microelectronics technology. The microelectronics technology is today referred to in a generic form as CMOS technology as this is the technology to fabricate Integrated circuits.
The CMOS term is well known in microelectronics field. In its wide meaning, it refers to the silicon technology for making integrated circuits. CMOS ensures very high accuracy of processing identical transistors (up to billions), high volume manufacturing, very low cost and high reproducibility at different levels (wafer level, wafer to wafer, and lot to lot). CMOS comes with high standards in quality and reliability.
There are many books and articles describing CMOS and there are many variants of CMOS technologies and devices that can be fabricated using CMOS technology. A very basic reference to CMOS can be found in Wikipedia (https://en.wikipedia.org/wiki/CMOS):
“Complementary metal-oxide-semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analogue circuits such as image sensors (CMOS sensor), data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1963 (U.S. Pat. No. 3,356,858). Besides digital applications, CMOS technology is also used in analogue applications. For example, there are CMOS operational amplifier ICs available in the market. Transmission gates may be used instead of signal relays. CMOS technology is also widely used for RF circuits all the way to microwave frequencies, in mixed-signal (analogue and digital) applications.”
Today CMOS processes exist in large microelectronics foundries, most of which are accessible to customers (i.e. can be accessed by more than one company, legal entity or individual). Such processes come with warranties and can be deployed in extremely large volumes.
Not all silicon technologies are CMOS technologies. Examples of non-CMOS technologies are given below:                Lab technologies (as opposed to foundry technologies) which are not deployable in volume and are not used for fabrication of a large number of transistors, but are specialised, used in low volume (such as those for R&D)        Screen printing technologies        Bio-technologies as for example those employed in making fluidic channels        MEMS technologies, such as those used for accelerometers or gyroscopes        Very high voltage vertical power device technologies, such as Vertical Bipolar Junction Transistor technologies        Technologies that use materials which are not CMOS compatible, such as gold, platinum or radioactive materials.        
It is worth mentioning that some of the technologies listed above as non-CMOS can in some cases be employed in post-CMOS or pre-CMOS processes without affecting the CMOS processes.
As already mentioned, most of the sensors are not fabricated in CMOS technology and many are not compatible with CMOS technology. For example, platinum is used as a heater in some of the previously mentioned reports, but is not available as a material in CMOS processes. CMOS technology offers many advantages such as low fabrication cost in high volume, possibility of circuit integration on the same chip, and good reproducibility from device to device. These advantages are not available if incompatible materials or processes are used.
There are some reports of CMOS based micro-hotplates. For example, Suehle et. al. “Tin Oxide Gas Sensor Fabricated Using CMOS Micro-hotplates and In-Situ Processing,” IEEE Electron Device Letters 1993, F. Udrea et. al. “Design and simulations of SOI CMOS micro-hotplate gas sensors,” Sensors and Actuators B 2001, M. Afridi Et. al, “A monolithic CMOS Microhotplate-Based Gas Sensor System,” IEEE Sensors Journal 2002, U.S. Pat. No. 5,464,966, M. Graf “CMOS microhotplate sensor system for operating temperature up to 500° C.” Sensors and Actuators B 2005, S. Z. Ali Et. Al, “Tungsten-Based SOI Microhotplates for Smart Gas Sensors” Journal of MEMS 2008, all report different examples of micro-hotplates fabricated in CMOS technology. Other reports by these same groups give information on similar devices, using polysilicon, MOSFETs, Single Crystal Silicon, and tungsten as heater materials.
One critical aspect for resistive gas sensors is the fabrication of the electrodes on top of the membrane which are used to measure the resistance of the sensing material. Ideally these electrodes should be made from a noble metal such as gold or platinum. However, both of these metals are usually not available in a CMOS process. As a result, one option is to use a metal available in CMOS, such as aluminium, which has been reported in Suehle et. al. “Tin Oxide Gas Sensor Fabricated Using CMOS Micro-hotplates and In-Situ Processing,” IEEE Electron Device Letters 1993. However, aluminium forms aluminium oxide on its surface, and as a result does not make a good electrical contact with the sensing material.
Another option is to deposit gold or platinum in a separate post-CMOS process. This allows the devices to have good contact to the sensing material while having a CMOS process. However, this step is not as straight forward as it seems. One possible method is to fabricate the device in CMOS and bulk etch it to release the membrane. Then spin photoresist on the wafer, and pattern using an appropriate mask, followed by sputtering or evaporation of the required metal.
However, this results in low yield and poor manufacturability because the photoresist coating and patterning can damage the membranes. Indeed the membranes are very thin (<10 μm), so when a photoresist is spun at high speed, there is likelihood that the membranes can break. The vacuum on the photoresist spinners can also cause membranes to break. Additionally, the fabricated membranes are often slightly bent due to residual stresses within the membrane layers. This can result in an uneven distribution of the photoresist, and also cause the mask patterns to be less well defined.
In addition, in general most foundries as well as lithography equipment are equipped and designed to work with standard wafers, and are not designed for working with wafers that have membranes. For example some mask aligners use a vacuum from the front side to pick up the wafer, which in the case of membrane wafers can cause several membranes to break. As a result of all these reasons, it is very difficult to deposit gold in this method on the micro-hotplates.
The other way to deposit is by performing the bulk etching after the patterning and deposition of the electrodes. For example M. Graf et. al. “CMOS microhotplate sensor system for operating temperature up to 500° C.” Sensors and Actuators B 2005 use this method to deposit the platinum electrodes. In this way, the patterning and gold deposition is done on a standard wafer, and the back etching step is done later. The problem with this method is that the platinum deposited on the wafer can contaminate the equipment used for the bulk etching.
M. Afridi Et. al, “A monolithic CMOS Microhotplate-Based Gas Sensor System,” IEEE Sensors Journal 2002, use a slightly different but similar process. In their process, after the CMOS processing, they deposit gold on the entire wafer, and then spin a resist on it and pattern it. The wafer is then bulk etched to release the membrane (a suspended membrane in this case). This is then followed by etching the gold to remove it from the regions where it is not required.
Using this method, the advantage is that no lithography or spinning is required on the wafers after bulk etching, so there are no problems with the handling of membranes in the lithography machines. However, similar to the method used by Graf, it still means that the wafers that are put in the machine for bulk etching already contain gold, and so can contaminate the machine. Additionally, the described device is a suspended membrane, which is mechanically less stable.
IR emitters are also well known. For example, Parameswaran et. al. “Micro-machined thermal emitter from a commercial CMOS process,” IEEE EDL 1991, and San et. al. “A silicon micromachined infra-red emitter based on SOI wafer” (Proc of SPIE 2007), describe IR emitter devices based on micro-hotplates on either suspended bridges or membranes.
To improve the emission of the IR emitters, often materials are deposited on top. This can be a coating of a high emissivity material, such as carbon, or metal blacks, or can be plasmonic structures. Plasmonic structures, such as those described by J. Daly “Nano-Structured Surfaces for Tuned Infrared Emission for Spectroscopic applications”, Micro- and Nano-photonic Materials and Devices, 2000, or by Y. Chang “Emission properties of Ag/dielectrid/Ag plasmonic thermal emitter with different lattice type, hole shape, and dielectric material,” Applied Physics letters 213102, 2009, can improve IR emission for certain wavelengths. These consist of one or more layers on the top surface specially designed in a repeating pattern.
These plasmonic structures can be made from gold or platinum, and their deposition method faces the same challenges as for the electrodes for the resistive gas sensors.
It is an object of the present invention to address the problems discussed above.